1. Field of the Invention
The present invention relates to a gas sensor for measuring oxides such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
In recent years, exhaust gas, which is discharged from vehicles or automobiles such as gasoline-fueled automobiles and diesel powered automobiles, contains nitrogen oxides (NOx) such as nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2), as well as carbon monoxide (CO), carbon dioxide (CO.sub.2), water (H.sub.2 O), hydrocarbon (HC), hydrogen (H.sub.2), oxygen (O.sub.2) and so on. In such exhaust gas, about 80% of the entire NOx is occupied by NO, and about 95% of the entire NOx is occupied by NO and NO.sub.2.
The three way catalyst, which is used to clean HC, CO, and NOx contained in the exhaust gas, exhibits its maximum cleaning efficiency in the vicinity of the theoretical air fuel ratio (A/F=14.6). If A/F is controlled to be not less than 16, the amount of produced NOx is decreased. However, the cleaning efficiency of the catalyst is lowered, and consequently the amount of discharged NOx is apt to increase.
Recently, in order to effectively utilize fossil fuel and avoid global warming, the market demand increases, for example, in that the discharge amount of CO.sub.2 should be suppressed. In order to respond to such a demand, it becomes more necessary to improve the fuel efficiency. In response to such a demand, for example, the lean burn engine and the catalyst for cleaning NOx are being researched. Especially, the need for a NOx sensor increases.
A conventional NOx analyzer has been hitherto known as an instrument for detecting NOx. The conventional NOx analyzer is operated to measure a characteristic inherent in NOx, based on the use of chemical luminous analysis. However, the conventional NOx analyzer is inconvenient in that the instrument itself is extremely large and expensive.
The conventional NOx analyzer requires frequent maintenance because optical parts are used to detect NOx. Further, when the conventional NOx analyzer is used, any sampling operation should be performed for measurement of NOx, wherein it is impossible to directly insert a detecting element itself into a fluid. Therefore, the conventional NOx analyzer is not suitable for analyzing transient phenomena such as those occur in the exhaust gas discharged from an automobile, in which the condition frequently varies.
In order to dissolve the inconveniences as described above, there has been suggested a sensor for measuring a desired gas component in exhaust gas by using a substrate composed of an oxygen ion-conductive solid electrolyte.
The suggested conventional gas sensor is exemplified by an all-range type oxygen sensor as shown in FIG. 7. Another type of a gas sensor for measuring gas (for example, NOx) having bound oxygen is also known, in which the oxygen concentration in the gas is lowered to be at a certain low level by using an oxygen pump, and then the oxygen concentration is further lowered to decompose NOx so that oxygen produced during the decomposition is measured by using an oxygen pump to measure NOx.
For example, the gas sensor shown in FIG. 7 will be explained. The gas sensor comprises an internal space 2 for introducing a measurement gas from an external space thereinto, a space (reference gas-introducing space) 4 for introducing a reference gas, for example, atmospheric air to be used as a reference for measuring oxides, and an oxygen pump 6 for maintaining a constant partial pressure of oxygen in the internal space 2.
The oxygen pump 6 comprises an inner pumping electrode 8 provided on the side of the internal space 2, an outer pumping electrode 10 provided on the side of the external space, and an oxygen ion-conductive solid electrolyte layer 12 existing between the both electrodes 8, 10. A reference electrode 14 is provided in the reference gas-introducing space 4.
The gas sensor performs feedback control for a control voltage Vp0 to be applied to the oxygen pump 6 so as to maintain a constant voltage of electromotive force (detection voltage V0) generated between the inner pumping electrode 8 and the reference electrode 14.
When the temperature of exhaust gas greatly changes as in automobiles, the gas sensor is provided with a heater 16 together with a mechanism provided for controlling electric power to be supplied to the heater 16.
In the conventional gas sensor, as shown in FIG. 7, in order to avoid oscillation of the feedback control system 18 concerning the oxygen pump 6, the inner pumping electrode 8 of the oxygen pump 6 is connected to a lead wire (GND) disposed on the negative side of the heater 16 to make a short circuit (see, for example, Japanese Laid-Open Patent Publication No. 3-167467).
However, when the wiring is arranged as described above, oxygen is moved from the inner pumping electrode 8 toward the heater 16 due to a leak current (heater leak current) from the lead wire of the heater 16.
That is, in the case of the conventional gas sensor, it is feared that the oxygen concentration in the internal space 2 is affected by the heater leak current, and the oxygen pump 6 fails to accurately control the oxygen concentration in the internal space 2.
When the current flowing through the heater 16 is large, the electromotive force, which is generated by the resistance of the lead wire of the heater 16, cannot be neglected. For example, assuming that a lead wire having a length of 3 m has a resistance value of 0.1 .OMEGA., when a heater current of 1 A is allowed to flow through the lead wire, an electromotive force (voltage) of "resistance value of lead wire x heater current=100 mV" is generated in the lead wire.
Therefore, the electromotive force is superimposed on the detection voltage V0 in the feedback control system 18, causing a problem that it is impossible to accurately control the control voltage Vp0 to be applied to the oxygen pump 6.
As described above, in the case of the conventional gas sensor, it is feared that the oxygen concentration in the internal space 2 cannot be controlled accurately, because the detection voltage V0 is affected by the length of the lead wire and the heater current (heater output), in the feedback control system 18 for the oxygen pump 6.